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  1. Mar 8, 2010 #1
    Hi all

    Is it possible that a RADAR speed camera, (the type that bounces off the road and is reflected off the vehicle and then the doppler shift represents the vehicle speed) can be affected by a raise in surface temperatue on a very hot day(40 degrees C plus, poster is in Australia)

    I believe that the temp of the road my affect the frequency of the RADAR signal giving false high readings.

    We have hundreds of people being fined and losiing their licences here, and it would appear that all GPS, cruise control and speedo in the state of Victoria are faulty, not the camera.

    Any help greatly appreciated
  2. jcsd
  3. Mar 8, 2010 #2


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    No, not to the extent that it would any practical effect (the speed of light does depend on the temperature of the air, but it is an extremely tiny effect).

    And, yes, GPS is not necessarily very accurate when it comes to measuring speed.
  4. Mar 8, 2010 #3


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    Is only one certain (new?) type of RADAR speed camera affected? Sometimes there has been faulty equipment introduced.

    What are your legal rights as a driver? Can you demand proof that the device was working correctly and has been calibrated according to rules? Ask a lawyer.

    Once the simple causes are excluded, one might look what effect the air temperature gradient has on the radar waves.
  5. Mar 8, 2010 #4


    My understanding is that a GPS is accurate to +/- 0.1% at 100 kph, when cruising at a steady speed, update lag not a problem then
  6. Mar 8, 2010 #5
    Thanks for the reply

    The type of camera in question is a Gatsometer Digital Radar System-Parabolic, it focuses a narrow beam on the centre of the lane at a 30 degree, angle the reflected beam from the vehicle is then processed looking at the doppler shift and the speed interpolated from this. As I understand it is supposed to be accuarate to +/- 3 kph so this is subtracted from the "Actual" measured speed. I was wondering if the road surface, being hot, could in some way affect the carrier frequency, I dont know how much the signal would change as a function of a vehicles speed, but i suspect, given the high carrier frequency, this would be quite small, thus a small error would give a false reading?
    Last edited: Mar 8, 2010
  7. Mar 8, 2010 #6


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    The radar is affected by the temperature of the air but as f95 said, it's parts per million.
    It's possible the unit itself could be out of calibration if it was designed for the Netherlands and is being operated at 50C in Oz - even if it wasn't you might be able to get off if the temperature was officially outside the unit spec.

    Generally fixed Gatso's don't clock you based on the radar - they take two photographs a fixed time apart and measure the distance you have gone. The radar is just to trigger the camera for likely 'victims'
  8. Mar 8, 2010 #7


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    What is the purpose of bouncing the beam off the ground at the 30 degree angle? Why not just bounce it off the car directly like most other radar speed guns?
  9. Mar 8, 2010 #8

    That's what the standard blurb says, as the poster above states it's just a trigger
  10. Mar 8, 2010 #9


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    Ah, thanks. I'd missed that -- we were posting at the same time.
  11. Mar 9, 2010 #10
    I know that here in the UK you can demand a copy of the most recent calibration certificate for the camera. There are certain rules governing this which your lawyer would be well aware of.
    If the camera is the usual Gatso type, as stated upthread the speed is measured from 2 photos taken at a fixed interval. The distance you travelled in that time is measured from the road markings.
    There have been concerns about the radar speed traps that actually do use the doppler effect; where the device is pointed at the front of the car. These are the hand held devices. The problem is the point of reflection on the car. If the device is not held steadily then the reflection point will move. If it started on the windscreen and then dropped to the front bumper, it would assume the speed of the car towards the operator to have been greater than it actually was.
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